全文获取类型
收费全文 | 5508篇 |
免费 | 228篇 |
国内免费 | 53篇 |
专业分类
测绘学 | 133篇 |
大气科学 | 443篇 |
地球物理 | 1273篇 |
地质学 | 2072篇 |
海洋学 | 406篇 |
天文学 | 948篇 |
综合类 | 18篇 |
自然地理 | 496篇 |
出版年
2021年 | 67篇 |
2020年 | 74篇 |
2019年 | 92篇 |
2018年 | 152篇 |
2017年 | 153篇 |
2016年 | 162篇 |
2015年 | 138篇 |
2014年 | 197篇 |
2013年 | 279篇 |
2012年 | 221篇 |
2011年 | 283篇 |
2010年 | 219篇 |
2009年 | 290篇 |
2008年 | 239篇 |
2007年 | 211篇 |
2006年 | 203篇 |
2005年 | 194篇 |
2004年 | 206篇 |
2003年 | 175篇 |
2002年 | 183篇 |
2001年 | 98篇 |
2000年 | 95篇 |
1999年 | 83篇 |
1998年 | 99篇 |
1997年 | 72篇 |
1996年 | 71篇 |
1995年 | 66篇 |
1994年 | 63篇 |
1993年 | 69篇 |
1992年 | 53篇 |
1991年 | 49篇 |
1990年 | 45篇 |
1989年 | 45篇 |
1988年 | 49篇 |
1987年 | 57篇 |
1986年 | 52篇 |
1985年 | 62篇 |
1984年 | 75篇 |
1983年 | 62篇 |
1982年 | 66篇 |
1981年 | 54篇 |
1980年 | 56篇 |
1979年 | 49篇 |
1978年 | 48篇 |
1977年 | 49篇 |
1976年 | 39篇 |
1975年 | 39篇 |
1974年 | 41篇 |
1973年 | 36篇 |
1972年 | 31篇 |
排序方式: 共有5789条查询结果,搜索用时 349 毫秒
21.
Haydon P. Mort Thierry Adatte Gerta Keller David Bartels Karl B. Fllmi Philipp Steinmann Zsolt Berner E.H. Chellai 《Cretaceous Research》2008,29(5-6):1008
With a multi-proxy approach, an attempt was made to constrain productivity and bottom-water redox conditions and their effects on the phosphorus accumulation rate at the Mohammed Plage section on the Tarfaya coast, Morocco, during the Cenomanian-Turonian Anoxic Event (OAE 2). A distinct δ13Corg isotope excursion of +2.5‰ occurs close to the top of the section. The unusually abrupt shift of the isotope excursion and disappearance of several planktonic foraminiferal species (e.g. Rotalipora cushmani and Rotalipora greenhornensis) in this level suggests a hiatus of between 40–60 kyrs at the excursion onset. Nevertheless, it was possible to determine both the long-term environmental history as well as the processes that took place immediately prior to and during OAE 2. TOC% values increase gradually from the base of the section to the top (from 2.5% to 10%). This is interpreted as the consequence of a long-term eustatic sea-level rise and subsidence causing the encroachment of less oxic waters into the Tarfaya Basin. Similarly a reduction in the mineralogically constructed ‘detrital index’ can be explained by the decrease in the continental flux of terrigenous material due to a relative sea-level rise. A speciation of phosphorus in the upper part of the section, which spans the start and mid-stages of OAE 2, shows overall higher abundances of Preactive mass accumulation rates before the isotope excursion onset and lower values during the plateau. Due to the probable short hiatus, the onset of the decrease in phosphorus content relative to the isotope excursion is uncertain, although the excursion plateau already contains lower concentrations. The Corg/Ptotal and V/Al ratios suggest that this reduction was mostly likely caused by a decrease in the available bottom oxygen content (probably as a result of higher productivity) and a corresponding fall in the phosphorus retention ability of the sediment. Productivity appears to have remained high during the isotope plateau possibly due to a combination of ocean-surface fertilisation via increased aridity (increased K/Al and Ti/Al ratios) and/or higher dissolved inorganic phosphorus content in the water column as a result of the decrease in sediment P retention. The evidence for decreased P-burial has been observed in many other palaeoenvironments during OAE 2. Tarfaya's unique upwelling paleosituation provides strong evidence that the nutrient recycling was a global phenomenon and therefore a critical factor in starting and sustaining OAE 2. 相似文献
22.
Thomas B. Boving Mark H. Stolt Janelle Augenstern Brian Brosnan 《Environmental Geology》2008,55(3):571-582
The control of polluted surface runoff and the assessment of possible impacts on groundwater is a concern at the local and
regional scale. On this background, a study investigates possible impacts of organic and inorganic pollutants (including bacteria)
originating from a permeable asphalt parking lot on the water quality immediately beneath it. The functioning of the permeable
pavement, including clogging and restricted vertical percolation, was also evaluated. Four nested sample ports (shallow and
deep) were installed below low- and high-traffic areas, including one port outside the parking lot. At least initially there
was a good hydraulic connection between the parking surface and the shallow sample ports. The presence of a geotextile layer
at the base of the parking lot structure, however, was identified in lab tests as one factor restricting vertical percolation
to the deeper ports. Clogging of the permeable surface was most pronounced in heavy traffic areas and below snow pile storage
areas. Corroborated by high electric conductivity and chloride measurements, sand brought in by cars during winter was the
principal cause for clogging. No bacteria or BOD were found in percolating water. Polycyclic aromatic hydrocarbons (PAH) were
present at concentrations near minimum detection limit. Nutrients (nitrate and phosphate) were being leached into the ground
via the permeable parking lot surface at annual flux rates of 0.45–0.84 g/m2/year. A multi-species tracer test demonstrated a retention capacity of the permeable parking lot structure of >90% for metals
and 27% for nutrients, respectively. 相似文献
23.
V. B. Naumov V. S. Kamenetsky R. Thomas N. N. Kononkova B. N. Ryzhenko 《Geochemistry International》2008,46(6):554-564
Melt inclusions were studied in chrome diopside from the Inagli deposit of gemstones in the Inagli massif of alkaline ultrabasic rocks of potassic affinity in the northwestern Aldan shield, Yakutia, Russia. The chrome diopside is highly transparent and has an intense green color. Its Cr2O3 content varies from 0.13 to 0.75 wt %. Primary and primary-secondary polyphase inclusions in chrome diopside are dominated by crystal phases (80–90 vol %) and contain aqueous solution and a gas phase. Using electron microprobe analysis and Raman spectroscopy, the following crystalline phases were identified. Silicate minerals are represented by potassium feldspar, pectolite [NaCa2Si3O8(OH)], and phlogopite. The most abundant minerals in the majority of inclusions are sulfates: glaserite (aphthitalite) [K3Na(SO4)2], glauberite [Na2Ca(SO4)2], aluminum sulfate, anhydrite (CaSO4), gypsum (CaSO4 × 2H2O), barite (BaSO4), bloedite [Na2Mg(SO4)2 × 4H2O], thenardite (NaSO4), polyhalite [K2Ca2Mg(SO4)4 × 2H2O], arcanite (K2SO4), and celestite (SrSO4). In addition, apatite was detected in some inclusions. Chlorides are probably present among small crystalline phases, because some analyses of aggregates of silicate and sulfate minerals showed up to 0.19–10.3 wt % Cl. Hydrogen was identified in the gas phase of polyphase inclusions by Raman spectroscopy. The composition of melt from which the chrome diopside crystallized was calculated on the basis of the investigation of silicate melt inclusions. This melt contains 53.5 wt % SiO2, considerable amounts of CaO (16.3 wt %), K2O (7.9 wt %), Na2O (3.5 wt %), and SO3 (1.4 wt %) and moderate amounts of Al2O3 (7.5 wt %), MgO (5.8 wt %), FeO (1.1 wt %), and H2O (0.75 wt %). The content of Cr2O3 in the melt was 0.13 wt %. Many inclusions were homogenized at 770–850°C, when all of the crystals and the gas phase were dissolved. The material of inclusions heated up to the homogenization temperature became heterogeneous even during very fast quenching (two seconds) producing numerous small crystals. This fact implies that most of the inclusions contained a salt (rather than silicate) melt of sulfate-dominated composition. Such inclusions were formed from salt globules (with a density of about 2.5 g/cm3) occurring as an emulsion in the denser (2.6 g/cm3) silicate melt from which the chrome diopside crystallized. 相似文献
24.
The current study provides an investigation of abiotic reduction of an oversaturated uranyl solution driven by iron nanoparticle oxidation. The reactivity of nano-scale zero-valent iron (ZVI) under mildly oxic conditions (1.2% O2 and 0.0017% CO2) was studied in 1000 ppm uranyl solution in the pH range 3-7, at reaction times from 10 min to 4 h. Reductive precipitation of UO2 was observed as the main process responsible for the removal of uranium from solution with the kinetics of reaction becoming increasingly favourable at higher pH. Despite working with an oversaturated uranium solution, the precipitation of UO2 occurred in preference to precipitation of UO3·2H2O (metaschoepite) at reaction times between 1 and 4 h and for uranyl solutions initially set up at pH ?5. Characterisation of both solid and solution phases was performed using X-ray photoelectron spectroscopy (XPS), focused ion beam (FIB) imaging, X-ray diffraction (XRD) and inductively coupled plasma atomic emission spectroscopy (ICP-AES). 相似文献
25.
Jacqueline L. Mann Christopher A. Shuman Karl J. Kreutz 《Geochimica et cosmochimica acta》2008,72(15):3907-3927
δ34S and sulfate concentrations were determined in snow pit samples using a thermal ionization mass spectrometric technique capable of 0.2‰ accuracy and requires ≈5 μg (0.16 μmol) natural S. The technique utilizes a 33S-36S double spike for instrumental mass fractionation correction, and has been applied to snow pit samples collected from the Inilchek Glacier, Kyrgyzstan and from Summit, Greenland. These δ34S determinations provide the first high-resolution seasonal data for these sites, and are used to estimate seasonal sulfate sources. Deuterium (δD) and oxygen (δ18O) isotope data show that the Inilchek and Summit snow pit samples represent precipitation over ≈20 months.The δ34S values for the Inilchek ranged from +2.6 ± 0.4‰ to +7.6 ± 0.4‰ on sample sizes ranging from 0.3 to 1.8 μmol S. δ34S values for Greenland ranged from +3.6 ± 0.7‰ to +13.3 ± 5‰ for sample sizes ranging from 0.05 to 0.29 μmol S. The concentration ranged from 92.6 ± 0.4 to 1049 ± 4 ng/g for the Inilchek and 18 ± 9 to 93 ± 6 ng/g for the Greenland snow pit. Anthropogenic sulfate dominates throughout the sampled time interval for both sites based on mass balance considerations. Additionally, both sites exhibit a seasonal signature in both δ34S and concentration. The thermal ionization mass spectrometric technique has three advantages compared to gas source isotopic methods: (1) sample size requirements of this technique are 10-fold less permitting access to the higher resolution S isotope record of low concentration snow and ice, (2) the double spike technique permits δ34S and S concentration to be determined simultaneously, and (3) the double spike is an internal standard. 相似文献
26.
Laura E. Wasylenki Bryan A. Rolfe Thomas G. Spiro 《Geochimica et cosmochimica acta》2008,72(24):5997-6005
The Mo stable isotope system is being applied to study changes in ocean redox. Such applications implicitly assume that Mo isotope fractionation in aqueous systems is relatively insensitive to frequently changing environmental variables such as temperature (T) and ionic strength (I). A major driver of fractionation is the adsorption of Mo to Mn oxyhydroxide surfaces [Barling J. and Anbar A. D. (2004) Molybdenum isotope fractionation during adsorption by manganese oxides. Earth Planet. Sci. Lett.217(3-4), 315-329]. Here, we report the results of experiments that determine the extent to which Mo isotope fractionation during adsorption of Mo to the Mn oxyhydroxide mineral birnessite is sensitive to T and I. The results are compared to new predictions from quantum chemical computations. We measured fractionation from 1 to 50 °C at I = 0.1 m and found that Δ97/95Modissolved-adsorbed varies from 1.9‰ to 1.6‰ over this temperature range. Experiments were also performed at 25 °C in synthetic seawater (I = 0.7); fractionation at this condition was the same within analytical error as in low ionic strength experiments. These findings confirm that the Mo isotope fractionation during adsorption to Mn oxyhydroxides is relatively insensitive to variations and T and I over environmentally relevant ranges. To relate these findings to potential mechanisms of Mo isotope fractionation, we also report results for density functional theory computations of the fractionation between and various possible structures of molybdic acid as a function of temperature. Because no plausible species fractionates from with a magnitude matching the experiments, we are left with three possibilities to explain the fractionation: (1) solvation effects on the vibrational frequencies of aqueous species considered thus far are significant, such that our calculations in vacuo yield inaccurate fractionations; (2) a trace aqueous species not yet considered fractionates from and then adsorbs to birnessite; or (3) a surface complex not present in solution forms on birnessite in which Mo is not tetrahedrally coordinated. Our findings help validate assumptions underlying paleoceanographic applications of the Mo isotope system and also lead us closer to understanding the mechanism of isotope fractionation during adsorption of Mo to Mn oxyhydroxides. 相似文献
27.
Microbial mass-dependent fractionation of chromium isotopes 总被引:1,自引:0,他引:1
Eric R. Sikora Thomas M. Johnson Thomas D. Bullen 《Geochimica et cosmochimica acta》2008,72(15):3631-3641
Mass-dependent fractionation of Cr isotopes occurs during dissimilatory Cr(VI) reduction by Shewanella oneidensis strain MR-1. Cells suspended in a simple buffer solution, with various concentrations of lactate or formate added as electron donor, reduced 5 or 10 μM Cr(VI) to Cr(III) over days to weeks. In all nine batch experiments, 53Cr/52Cr ratios of the unreacted Cr(VI) increased as reduction proceeded. In eight experiments covering a range of added donor concentrations up to 100 μM, isotopic fractionation factors were nearly invariant, ranging from 1.0040 to 1.0045, with a mean value somewhat larger than that previously reported for abiotic Cr(VI) reduction (1.0034). One experiment containing much greater donor concentration (10 mM lactate) reduced Cr(VI) much faster and exhibited a lesser fractionation factor (1.0018). These results indicate that 53Cr/52Cr measurements should be effective as indicators of Cr(VI) reduction, either bacterial or abiotic. However, variability in the fractionation factor is poorly constrained and should be studied for a variety of microbial and abiotic reduction pathways. 相似文献
28.
David P. Bacon Nash’at N. Ahmad Thomas J. Dunn Michael C. Monteith Ananthakrishna Sarma 《Natural Hazards》2008,44(3):317-327
By definition, a crisis is a situation that requires assistance to be managed. Hence, response to a crisis involves the merging
of local and non-local emergency response personnel. In this situation, it is critical that each participant: (1) know the
roles and responsibilities of each of the other participants; (2) know the capabilities of each of the participants; and (3)
have a common basis for action. For many types of natural disasters, this entails having a common operational picture of the unfolding events, including detailed information on the weather, both current and forecasted, that may impact on either
the emergency itself or on response activities. The Consequences Assessment Tool Set (CATS) is a comprehensive package of
hazard prediction models and casualty and damage assessment tools that provides a linkage between a modeled or observed effect
and the attendant consequences for populations, infrastructure, and resources, and, hence, provides the common operational
picture for emergency response. The Operational Multiscale Environment model with Grid Adaptivity (OMEGA) is an atmospheric
simulation system that links the latest methods in computational fluid dynamics and high-resolution gridding technologies
with numerical weather prediction to provide specific weather analysis and forecast capability that can be merged into the
geographic information system framework of CATS. This paper documents the problem of emergency response as an end-to-end system
and presents the integrated CATS–OMEGA system as a prototype of such a system that has been used successfully in a number
of different situations. 相似文献
29.
G. Diego Gatta Nicola Rotiroti Martin Fisch Milen Kadiyski Thomas Armbruster 《Physics and Chemistry of Minerals》2008,35(9):521-533
The elastic and structural behaviour of the synthetic zeolite CsAlSi5O12 (a = 16.753(4), b = 13.797(3) and c = 5.0235(17) Å, space group Ama2, Z = 2) were investigated up to 8.5 GPa by in situ single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions. No phase-transition occurs within the P-range investigated. Fitting the volume data with a third-order Birch–Murnaghan equation-of-state gives: V 0 = 1,155(4) Å3, K T0 = 20(1) GPa and K′ = 6.5(7). The “axial moduli” were calculated with a third-order “linearized” BM-EoS, substituting the cube of the individual lattice parameter (a 3, b 3, c 3) for the volume. The refined axial-EoS parameters are: a 0 = 16.701(44) Å, K T0a = 14(2) GPa (βa = 0.024(3) GPa?1), K′ a = 6.2(8) for the a-axis; b 0 = 13.778(20) Å, K T0b = 21(3) GPa (βb = 0.016(2) GPa?1), K′ b = 10(2) for the b-axis; c 0 = 5.018(7) Å, K T0c = 33(3) GPa (βc = 0.010(1) GPa?1), K′ c = 3.2(8) for the c-axis (K T0a:K T0b:K T0c = 1:1.50:2.36). The HP-crystal structure evolution was studied on the basis of several structural refinements at different pressures: 0.0001 GPa (with crystal in DAC without any pressure medium), 1.58(3), 1.75(4), 1.94(6), 3.25(4), 4.69(5), 7.36(6), 8.45(5) and 0.0001 GPa (after decompression). The main deformation mechanisms at high-pressure are basically driven by tetrahedral tilting, the tetrahedra behaving as rigid-units. A change in the compressional mechanisms was observed at P ≤ 2 GPa. The P-induced structural rearrangement up to 8.5 GPa is completely reversible. The high thermo-elastic stability of CsAlSi5O12, the immobility of Cs at HT/HP-conditions, the preservation of crystallinity at least up to 8.5 GPa and 1,000°C in elastic regime and the extremely low leaching rate of Cs from CsAlSi5O12 allow to consider this open-framework silicate as functional material potentially usable for fixation and deposition of Cs radioisotopes. 相似文献
30.
贵州遵义牛蹄塘组黑色岩系的硒同位素变化及其环境指示初探 总被引:4,自引:0,他引:4
西南早寒武世牛蹄塘组是分布在中国南方扬子地台的一套黑色岩系,区域上发育有典型富集多金属元素的硫化物矿化层.选取遵义松林小竹牛蹄塘组下部富硒、钼、镍等元素的黑色岩系剖面,利用Se同位素初步探讨了该剖面岩石的沉积环境和硒的可能来源.结果表明,小竹牛蹄塘组下部剖面黑色岩系的δ82/76SeSRM3149比值变化较大,下部底层含碳斑脱岩与磷块岩的变化范围在-4.35‰~ 4.11‰之间;中间镍钼层及碳质页岩、碳质碳酸盐岩的δ82/76SeSRM3149变化范围窄,平均值为0.9‰±0.23‰(n=4);上层碳质页岩为-1.24‰.结合已发表的钼同位素数据,认为中间岩石沉积于缺氧/无氧环境,但存在盆地海水与热液或充氧水团的混合;下层的岩石曾一度位于充氧与贫氧环境的边界面,局部岩石曾暴露地表经历了较强的风化和蚀变作用,海水中硒有可能来自底部富硒斑脱岩的氧化淋滤或海底热液.据此推测遵义松林小竹牛蹄塘组下部岩石的沉积环境极可能处在局限盆地靠近陆地的边缘部分,经历了充氧→贫氧→缺氧/无氧→贫氧的演化阶段. 相似文献